1. Technical Field
This document relates to an osmotic pump for forward osmosis devices.
2. Background
Recently, water purifiers employing forward osmosis (FO) have been commercialized. In FO devices concentrated nutrient syrup or powder is contacted to one side of a semi-permeable membrane and dirty or salty water is contacted to the other side. The membranes have a molecular selectivity similar to reverse osmosis membranes, which allow water to pass but effectively block dissolved salts, sugars, and contaminants, as well as biologicals such as viruses, bacteria, cysts, pyrogens, and prions. Unlike the pressure driven reverse osmosis process, FO draws pure water from contaminated water by osmosis. No pumping is required; a concentrated nutrient solution passively pulls water across the membrane from the dirty water, creating a dilute drink which supplies needed calories, electrolytes and hydration in a remote location or during an emergency.
Commercial devices have two basic designs; batch or continuous production of drink. The batch designs use powder or syrup in a membrane bag. The bag must be allowed to hydrate in the source water until the desired amount of water has been absorbed after which the drink may be consumed.
Continuous production devices use membrane elements that have a large amount of membrane sealed so that: (1) there is a syrup inlet; (2) there are flow paths which bring the syrup into contact with one side of a membrane and source water into contact with the other side; (3) there is an exit for the dilute drink; and (4) there is a method for introducing syrup at a rate so that a desired dilution of the drink occurs in the element.
However, the methods for controlling syrup rates involve either highly complex and/or power-consuming control systems. In case of disaster areas or remote sites, such power sources may not be available and technically trained individuals may also not be available to support such FO-based drinking water systems.
A common application of the device introduces “sports drink” syrup at a concentration of 65% solids (35% water by weight) at a rate so that in the element the syrup absorbs from 15 to 50 times its volume in water. To keep the systems simple, inexpensive and avoid power consumption, in some devices the rate of syrup flow is controlled by either an IV drip device or by gravity flow through a capillary tube.
However, with these methods it is difficult to achieve a standard dilution because: (1) the rate of water transfer through the membrane is temperature dependent, such that water at cold temperatures is transferred more slowly than at warm temperatures; (2) the rate of water transfer is dependent on dissolved solids in the source water, such that dissolved solids in the dirty water slow water transfer; (3) the water transfer through the membrane declines over months of use; and (4) changes in syrup viscosity with temperature affect the syrup flow in a capillary tube.
The result is that such variables as temperature, dissolved solids in the source water, and age of the membranes all cause significant changes in the osmotic strength (such as nutrient or salt) of the resulting drink produced.